The present invention relates to a rare gas discharge lamp manufactured by sealing a rare gas containing xenone gas as its main component into a tubular glass bulb in which a phosphor film is coated on its inner wall and by forming a pair of belt-shaped electrodes on the outer wall of the glass bulb.
In general, since a rare gas discharge lamp of this type has a small outer diameter and is of a rare gas discharge type, its brightness or discharge voltage is hardly influenced by an ambient temperature, and its service life is long. Therefore, the rare gas discharge lamp has attracted attention as an original reading light source of OA equipment such as a facsimile apparatus or an OCR or a back light of a liquid crystal display device.
In a conventional rare gas discharge lamp, however, as disclosed in U.S. Pat. No. 4,899,090, a pair of electrodes are enclosed at two ends of an elongated glass bulb, and a belt-shaped auxiliary electrode is formed in contact with the outer wall of the glass bulb between the two electrodes, thereby moving a positive column toward the auxiliary electrode side. Therefore, although a phosphor film near the auxiliary electrode can be effectively excited, it is difficult to efficiently excite the entire phosphor film. As a result, a bright discharge lamp cannot be easily obtained.
Japanese Patent Laid-Open No. 60-12660 discloses a fluorescent lamp in which mercury vapor is sealed into a glass bulb, and a pair of electrodes having various shapes such as a ring are formed on the outer wall of the glass bulb, thereby generating discharge in the bulb. In this fluorescent lamp, the electrodes are formed on the outer wall of the glass bulb to suppress sputtering caused by evaporation of an electrode material in the bulb. As a result, a reduction in luminous intensity is prevented to realize a long service life. However, since the mercury vapor is used as a discharge gas, not only the luminous intensity is low, but also a portion of a phosphor film opposing the electrode is damaged and degraded by mercury ions, thereby reducing a luminous flux. Therefore, since the luminous intensity of this fluorescent lamp is insufficient and its deterioration over time is large, it is difficult to use the fluorescent lamp as a light source of OA equipment.